1. Field of the Invention
This invention relates to a support section structure for a swing arm for supporting a rear wheel of a motorcycle for a rocking motion in upward and downward directions.
2. Description of Background Art
Some motorcycles adopt a drive shaft as a transmission section for providing power from an engine mounted on the motorcycle.
In the motorcycle which adopts the drive shaft, an output power shaft of the engine carried on a vehicle body frame and the drive shaft supported on a swing arm are connected to each other through a universal joint. Since the universal joint section is bent in response to an upward or downward rocking motion of the swing arm, the universal joint is disposed coaxially on a pivotally supporting shaft for the swing arm.
As one of support section structures for a swing arm of such a motorcycle described above, a support section structure is known wherein a pair of pivotally supporting walls are provided in a spaced relationship from each other in the vehicle widthwise direction at positions of a vehicle body frame on the rearward side of an engine and a swing arm is supported at a front end portion thereof between the pivotally supporting walls while a drive shaft (universal joint) is disposed on the outer side of one of the pivotally supporting walls in the vehicle widthwise direction. See, Japanese Patent Laid-Open No. 2002-87364. In this instance, since the drive shaft is disposed on the outer side in the vehicle widthwise direction with respect to the pivotally supporting shaft for the swing arm, a front end portion of the swing arm is supported on the vehicle body frame in such a relationship that it is displaced to one of the left and right sides of the vehicle body frame (on the side opposite to the drive shaft). In addition, on the vehicle body frame side, a pair of pivotally supporting walls are formed in a displaced relationship to one of the left and right sides. It is to be noted that, in the case of the support section structure for a swing arm disclosed in Japanese Patent Laid-Open No. 2002-87364, a cover member for covering the outer sides of the drive shaft in the vehicle widthwise direction is removably mounted on the vehicle body frame.
However, with the conventional support section structure for a swing arm, it is difficult to sufficiently assure the distance between the pair of pivotally supporting walls of the vehicle body frame on the opposite sides because the pivotally supporting walls are displaced to one side in the vehicle widthwise direction and the drive shaft is disposed at a side portion of one of the pivotally supporting walls. Thus, in order to support the front end portion of the swing arm with high rigidity, it cannot be avoided to form the pivotally supporting walls in an increased size by increasing the thickness of the pivotally supporting walls or by a similar means. If the pivotally supporting walls have an increased size, then an additional member must be provided or an existing part must be formed which results in an increase in the thickness in order to adjust the weight balance in the leftward and rightward directions of the vehicle body with respect to the pivotally supporting walls of the increased size. As a result, there is the possibility that the weight of the entire vehicle body may be increased.
Further, in the case of the support section structure for a swing arm disclosed in Japanese Patent Laid-Open No. 2002-87364, while a cover member which covers the outer sides of the drive shaft in the vehicle widthwise direction is mounted on the vehicle body frame, this cover member is a removable cover member for the maintenance and does not contribute to an enhancement of the rigidity of the vehicle body frame.